A single crystal means a homogeneous material having one regular arrangement without any boundaries, contrary to materials in which general ceramic components exist in the form of powder and particles thereof disorderly exist in a state where the particles have boundaries. Due to such a regular physical state, the single crystal provides physical, electromechanical, optical and electrooptical properties completely different from those of general ceramics and thus is used for preparation of a variety of latest devices such as tunable filters for radio communication, surface acoustic wave (SAW) devices or film bulk acoustic resonator (FBAR) devices, optical modulators, medical and military ultrasonic transducers, nonvolatile memory devices, actuators for fine displacement control, small batteries, radiant energy receivers, and the like.
Particularly, a single crystal having the properties of intelligent relaxor ferroelectrics among such single crystals can be used as a sensor due to its intelligence, can be micro-machined due to its most dense, regular arrangement of atoms and molecules in a given space, and can exhibit superior piezoelectric, electromechanical and electrooptical properties due to the absence of a grain boundary. The term “relaxor” means the property of relaxing temperature dependency by which physical property values are affected by outside temperature, which is a critical problem that most electrical parts or devices, materials, equipment and the like have.
Due to such superior properties, the intelligent relaxor ferroelectric single crystal can be used for ultrasonic transducers, actuators, microphones, tunable filters for wire/wireless communications, SAW devices, FBAR and the like by using its piezoelectric property by which a voltage is generated upon application of stress to the single crystal and its electrostrictive property by which striction is generated upon application of a voltage thereto the other way around. Further, due to its excellent electrooptical property, the single crystal is a promising material for optical modulators, optical switches and the like that are functional devices for optical communication. Moreover, even in a nonvolatile memory field on which much interest has been focused recently in a memory field, the single crystal can be used to prepare superior memory devices due to its excellent ferroelectric property.
However, the single crystal that has superior properties and is expected to be applied to various latest devices as described above is generally very difficult to grow to a commercially applicable single crystal having a diameter of 5 cm (2 inches) or greater. Thus, the single crystal has not yet been commercialized heretofore. Even though such a single crystal is obtained, the quality of the single crystal is closely related to a method by which a ceramic material of the single crystal is prepared, and the composition of the ceramic material. Research reports that have been conducted heretofore are as follows.
G. A. Smolenskii and A. I. Agranovskaya first reported lead magnesium niobate (Pb(Mg1/3Nb2/3)O3; “PMN”), which is one of new relaxor ferroelectric ceramics, in Sov. Phys. Solid State Vol. 1, 1429 (1959). Since such a ceramic was first reported, W. A. Bonner and L. G. Van Uitert reported that they succeeded in growing a PMN single crystal to a size of a several mm to 2 cm using the Kyropoulos method, in Mater Res. Bull. Vol. 2, 131 (1967). In 1980, N. Setter and L. E. Cross reported that they succeeded in growing a PMN single crystal to a size of 1 cm using a flux method, in J. Cryst. Growth, Vol. 50, p 555 (1980). In 1983, S. Nomura and K. Uchino reported various properties of such a PMN single crystal and the properties of a PMN-PT single crystal that is a PT compound of the PMN single crystal in Ferroelectrics Vol. 50, p 107-203 (1983). They reported that the single crystal was excellent in piezoelectric, electromechanical, ferroelectric and pyroelectric properties.
Even thereafter, many research results have been reported. Recently, Sang-goo Lee et al. first succeeded in growing PMN to a size of about 1 cm using the Bridgemann method and reported its superior physical properties (see Appl. Phys. Letts. Vol. 74, No. 7, 1030 (1999)). Further, Luo et al. reported the growth and the properties of a PMN-PT single crystal having a diameter of 2.5 cm using a supplemented Bridgemann method in 1999 IEEE Ultrasonic Symposium-1009.
However, as can be seen from the aforementioned, conventional PMN-PT compositions and processes of preparing the single crystals using the compositions, there have been reported and researched only processes of preparing single crystals having a diameter of about 2.5 cm that can be used only for researches or experiments. Such a size is much short of a diameter of 5 cm that is a minimum wafer size for use in a current device fabricating process. Therefore, the development of a device using such a single crystal has not been made. Even though the flux-growth method, which is a method exhibiting most excellent reproducibility among methods of preparing single crystals that have been reported heretofore, is employed, a single crystal having an irregular size is obtained. Thus, there is a disadvantage in that the method cannot be used for commercial mass production.
The major reason that such a relaxor ferroelectric single crystal has not yet been applied to commercial devices though studies on the relaxor ferroelectric single crystal have been continuously conducted for last 40 years is that it is impossible to stably produce a large single crystal having a diameter of about 5 cm by using the existing single crystal growth method, in view of the properties of the ferroelectric material containing a great deal of lead component.
Meanwhile, methods for solving the problem are disclosed in Korean Patent No. 0384442 and U.S. Pat. Nos. 6,491,899 and 6,468,498 in the name of and assigned to Ibule Photonics Co., Ltd., respectively. However, the methods also have problems in that composition uniformity is deteriorated in a single crystal due to a low phase-transition temperature, and its application range is relatively narrow.